I've really liked this plugin for quite some time and use it without any issues on another blog I manage. Just setup a new site for another project, and after installing it I am getting the following error:

Fatal error: Call to a member function session_begin() on a non-object in /home/survivor/public_html/forum/includes/functions.php on line 3814

Site is survival.us.com

Additional info:
The last run didn't finish! Maybe you can raise the memory or time limit for PHP scripts. Learn more
The last known memory usage of the script was 3.25MB, the limit of your server is 256M.
The last known execution time of the script was 0.04 seconds, the limit of your server is 120 seconds.
The script stopped around post number 31 (+/- 100)
If you changed something on your server or blog, you should rebuild the sitemap manually.
If you encounter any problems with the build process you can use the debug function to get more information.

I would appreciate it if anyone could point me in the right direction to fix the issue. I'd really like to use it on the new site. Thanks.